The most serious accident that may occur at a nuclear power station is a loss of coolant accident which means an instantaneous rupture of a pipeline of a maximum diameter and free escape of the coolant at both ends of that pipeline. The coolant brings radioactive fission products to the reactor room. The steam released by the ebullient coolant raises the pressure in the reactor room so that there is a danger of discharge of radioactive products into the atmosphere.
One of the ways to prevent radioactive contamination of the environment is the use of hermetically containments which hold the radioactive products released as a result of an accident.
Such containments are designed for a maximum steam pressure. A loss of coolant accident normally results in a release of a large amount of steam, so a containment must be large and sturdy in order to withstand the pressure of the high-temperature steam-and-air mixture. Clearly, such an containment is quite expensive.
One of the ways to cut down the cost of a containment is to reduce the pressure under it. The problem can be solved by using a cooling medium for steam condensation or by dividing the inner space of the containment into two compartments and arranging a condenser means between these compartments (cf. U.S. Pat. No. 3,379,613, Cl. 176/37).
The reactor plant and coolant circuit equipment are arranged in the first compartment; the second compartment is intended to receive air driven from the first compartment by high pressure due to the release of steam after a loss of coolant accident. Passive-type condensers are disposed between the first and second compartments (cf. also U.S. Pat. No. 3,253,996, Cl. 176/38).
Following an accident, the ebullient coolant produces steam that mixes with the air filling the first compartment; as a result, the pressure in the first compartment becomes higher than that in the second compartment. The pressure difference forces the steam-and-air mixture into the condenser which condenses the steam; the air is driven into the second compartment, and the pressure in that compartment goes up. The passive-type condenser is an ice condenser or a pool of water through which the steam-and-air mixture is bubbled. Despite the use of pressure reduction means, excess pressure within the containment persists over a long period of time. No containment can be 100% hermetic, so it is practically impossible to avoid radioactive contamination of the environment. A containment must be extremely tight to keep the contamination within permissible limits; this again means that such a contaimment must be quite expensive.
There is known a system for mitigating the consequences of a loss of coolant accident at a nuclear power station, which comprises two rooms whereof the first accommodates a reactor plant and coolant circuit and communicates through a valve with the second room where pressure is permanently kept below one atmosphere by sucking air out of that second room (cf. U.S. Pat. No. 3,375,162, Cl. 176/37).
Following a loss of coolant accident, the steam-and-air mixture is driven into the second room which accommodates a condenser of steam. The influx of the steam-and-air mixture raises the pressure in the second room. A sufficiently high degree of rarefaction in the second room is bound to bring about rarefaction in the first room. It must be pointed out, however, that the system under review is highly expensive because it necessitates the construction of said second chamber where a subatmospheric pressure has to be maintained all the time.
There is further known a system for mitigating the consequences of a loss of coolant accident at a nuclear power station. This system comprises a first room which accommodates a reactor plant and communicates with a second room intended to receive the air forced thereto from the first room where the pressure is increased after an accident and loss of coolant (cf. U.S. Pat. No. 4,056,436, Cl. 176/37).
Arranged between the first and second rooms is a passive-type condenser of steam released due to ebullition of the leaking coolant. The first room communicates with the second room through a channel having a check valve installed at the inlet of the second room and intended to prevent the backflow of air ousted from the first room. The condenser is a bubbler comprising at least one trough filled with a cooling liquid and covered by a jacket. The walls of the trough and jacket form channels for the steam-and-air mixture. The outlet of the jacket communicates with the check valve. In order to produce rarefaction in the first room after an accident, use is made of an active-type sprinkler means which is turned on when all the coolant is out and power supply is resumed, and is turned off when all the steam is condensed. The reliability of the system under review depends, in fact, on the reliability of the check valve installed at the inlet of the second room. The check valve is an active-type device in the sense that it comprises movable parts. The check valve may fail to operate if any of these parts is damaged or if a foreign object gets into the valve. As a result, the air from the second room flows back to the first room to raise pressure in the first room above the atmospheric one. High pressure in the first room may persist over a prolonged period of time so that there may be a leakage of radioactive products into the atmosphere through cracks that may develop in the walls of the first room.
There is still further known a system for mitigating the consequences of a loss of coolant accident at a nuclear power station. The system comprises a first room which accommodates a reactor plant and an active-type sprinkler means to condense steam produced by the ebullient coolant after an accident. As the steam rises pressure in said first room, air is driven from this room through holes provided in the walls of the room to an intermediate chamber and through channels t a basin-type condenser of steam, which is arranged in a second room (cf. U.S. Pat. No. 3,668,069, Cl. 176/38).
The holes in the walls of the first room are found at the upper part of the walls and spaced equidistantly. A loss of coolant accident may occur at any point of the reactor plant, and the steam released as a result of the accident reaches the intermediate chamber through the holes that are nearest to the coolant leakage point, so the rate at which the air is driven from the first room to the second is kept at a minimum. As the sprinkler means is brought into play, the pressure in the first room becomes lower than that in the second room; the water is driven out of the channels and the air from the second room is forced back to the first room. The system is such that a pressure above one atmosphere is maintained therein over a prolonged period of time following an accident, and radioactive products are released into the atmosphere through cracks in the containment.